1. Field of the Invention
This disclosure relates to a detection circuit to detect a short-to-ground fault (ground fault) or an open fault of an external terminal connected to a load, and relates to a load driving device and an electrical apparatus using the detection circuit.
2. Description of Related Art
A motor driving device is known to detect a current flowing through switching elements of an output stage and to maintain the current at a predetermined target value by chopping an ON period of the switching elements. An example of the motor driving device using the technique (i.e., a constant current chopping function) is disclosed in Japanese patent publication No. H11-206189.
Also a motor driving device is known to detect an over current flowing through switching elements of an output stage and then turns OFF the switching elements forcefully. Examples of motor driving devices using the technique (i.e., an over current protection function) are disclosed in Japanese patent publication No. H05-111144 and in Japanese patent publication No. H05-111145.
The constant current chopping function or the over current protection function of a conventional motor driving device may not be operated normally under specific motor driving conditions. This can result in an abnormal over heating of a motor, breakage of a motor or breakage of a motor driving device.
Each of FIG. 12A and FIG. 12B shows a diagram of a motor driving device in accordance with the related art (showing a H-bridge circuit of an output stage). In a first operating state shown in FIG. 12A, transistors FET1 to FET4 form a H-bridge circuit, the transistors FET1 and FET4 are turned ON, and the transistors FET2 and FET3 are turned OFF. In a second operating state shown in FIG. 12B, the transistors FET1 and FET4 are turned OFF, and the transistors FET2 and FET3 are turned ON.
According to the related art, when a coil current is driven by the motor driving device for each phase of the stepping motor, as the motor is rotating (e.g., while a stepping pulse is provided), the H-bridge circuit alternates between a first operating state and a second operating state every time a stepping pulse (not illustrated) is provided to the stepping motor. On the other hand, as the motor is not rotating (e.g., while a stepping pulse is stopped), the H-bridge circuit is held either in the first operating state or in the second operating state.
When a coil current is driven for a brushed DC motor by a motor driving device according to the related art, while the motor is rotating in a first direction (e.g., in a forward rotation), the state of the H-bridge circuit is held in the first operating state. On the other hand, while the motor is rotating in a second direction adverse to the first direction (e.g., in a reverse rotation), the state of the H-bridge circuit is held in the second operating state.
For a motor driving device in accordance with the above mentioned related art, the constant current chopping function is accomplished by detecting sink current flowing through the H-bridge circuit using a resistor RNF, and by maintaining the detected sink current at a predetermined target value by chopping an ON period of the transistors FET1 to FET4.
For a motor driving device in accordance with the above mentioned related art, the over current protection function is accomplished by detecting the current flowing through the transistors FET1 to FET4, and by turning OFF the transistors FET1 to FET4 forcefully when it is determined that an over current has occurred because even one of the detected currents exceeds the predetermined upper value (e.g., 4.5 A).
The following description regarding the related art is based on an assumption when a point Y (i.e., a low level output terminal of the H-bridge circuit operating as the first operation state) becomes a ground fault during the H-bridge circuit is kept as the first operation state. In this description, a ground fault means an abnormal state when an output terminal is short-circuited to a ground terminal or a low level potential.
When the H-bridge circuit is operating in the first operating state, if the point Y becomes the ground fault, a current Ia from a power source input terminal to a coil L1 via the transistor FET1 flows through to a ground terminal via a current path caused by the ground fault (i.e., not a current path via the transistor FET4 and the resistance RNF). In such a situation, the sink current does not flow through the resistor RNF of the H-bridge circuit, and the foregoing constant current chopping function does not work. Thus both transistors FET1 and FET4 are in the ON state (i.e., no chopping state). As a result, the voltage level of point X (i.e., a high level output terminal of the H-bridge circuit operating in the first operating state) becomes 18V (i.e., VCC), and the voltage level of point Y becomes 0V (i.e., GND).
At this time, as indicated by formula (1) below, a current value of the current Ia is adjusted by a wire wound resistance value RL of the coil L1, by the ON resistance Ron1 of the transistor FET1 (e.g., Ron=0.6 ohm) and by a power source voltage VCC (e.g., VCC=18V).Ia=VCC/(RL+Ron1)  (1)
If the wire wound resistance value RL of the coil L1 is a relatively small value and the current Ia exceeds the predetermined upper value (e.g. 4.5 A) when the current Ia is not restricted by the coil L1, the foregoing over current protection function works, and the transistors FET1 to FET4 are turned OFF forcefully. The function does not depend on whether or not current is flowing through the resistance RNF.
However, if the wire wound resistance value RL of the coil L1 is relatively large and the current Ia does not reach the predetermined upper value, the foregoing over current protection function does not work. As a result, the current Ia, which does not reach the over current protection value, flows through the motor driving device continuously. This results in abnormal overheating or breakage of the motor or breakage of the motor driving device. The possibility of breakage mostly increases if the current Ia continues to flow through, such that the current value reaches an absolute maximum rating value and does not reach the over current protection value.
When the H-bridge circuit is converted from the first operating state to the second operating state (i.e., in the case of the rotating state of the stepping motor, or in case of the converting state from forward rotation to reverse rotation of the brushed DC motor), a current Ib flowing from the power source terminal to the ground terminal via the transistor FET2 does not flow via the coil L1, the transistor FET3 and the resistance RNF. The current Ib flowing from the power source terminal to the ground terminal via the transistor FET2 flows thorough a current path caused by the ground fault.
Thus, as indicated by formula (2) below, the current Ib does not flow through the coil L1, and the value of the current Ib is adjusted by the ON resistance Ron2 of the transistor FET2 (e.g., Ron=0.6 ohm) and a power source voltage VCC (e.g., VCC=18V).Ib=VCC/Ron2=18/0.6=30A  (2)
A value of the current Ib is limited to a small value calculated by the foregoing formula (2) according to the current driving capability of the power source or the current driving capability of the transistor FET2. Nevertheless, the value of the current Ib exceeds the predetermined value (e.g., 4.5 A) considerably, and the foregoing over current protection function works. Thus, the transistors FET1 to FET4 are turned OFF forcefully.
As described above, when using the motor driving device of the related art under certain conditions (e.g., when a point Y becomes ground fault while the H-bridge circuit is maintained in the first operating state), the constant current chopping function or the over current protection function do not work properly. This can result in an abnormal overheating or breakage of the motor or breakage of the motor driving device.
Furthermore, apart from the ground fault, when using the motor driving device of the related art, there is a possibility of a connection detach occurring between the motor driving device and the motor (i.e., open fault). In such a case, the motor is no longer able to operate properly, though there is an electrical apparatus which continues to operate properly after functioning of one of the motors has been stopped.
For example, if there is an open fault with a motor for a toner box to withdraw the used toner in a copy machine, the copy function continues to operate normally until the toner box withdraws the used toner is fulfilled with the used toner. Also, if there is an open fault with a motor for toner distribution, the print function is able to continue to operate for some time without an affection to the printed out characters.
With respect to the above mentioned copy machine, the open fault can be detected after some time for the first time as a toner withdrawing error or with a printed character error. In other words, for the foregoing copy machine, there is a possibility of causing fatal damage to the entire machine caused by a normal operation after the motor has been abnormally stopped.
Also, it is difficult to identify a motor of an abnormal state for the electrical apparatus with multiple motors. It requires time for maintenance, and there can be situations in which it is not possible to identify the broken portion; exchange for the entire circuit board may be required.
Load driving devices which include a H-bridge circuit as an output stage can have same kind of problem as described above with respect to the motor driving device.